Amplification and selectivity control circuit



Jan. 19, 1937. RUST 2,068,1 12

AMPLIFICATION AND SELECTIVITY CONTROL CIRCUIT Filed Aug. 20, 1955 mowmmumw (HANGER Mean/us 0F a/as\ Mmm/s a; 41m: Sal/22:5

INVENTOR. NOEL M RUST ATTORNEY.

Patented Jan. 19, 1937 UNITED STATES AMPLIFICATION AND SELECTIVITYCONTROL CIRCUIT Nol Meyer Rust, Chelmsford, England, assignor to RadioCorporation of America, a corporation of Delaware Application August 20,1935, Serial No. 36,964 In Great Britain August 15, 1934 1 Claim.

This invention relates to thermionic valve circuit arrangements suitablefor use in radio receivers and in like apparatus.

More particularly the invention relates to ther- 5 mionic valve circuitarrangements for carrier frequency operation and of the kind whereinamplification and selectivity are increased by the use of reaction.

The principal object of the invention is to provide an improved highfrequency thermionic valve circuit arrangement of the reaction typewhereby very high selectivity and stability is obtained with the use ofcomparatively simple apparatus.

Another important object of the invention is to provide a stable simplehigh frequency thermionic valve circuit arrangement of the reaction typewherein it is possible to obtain, either manually or automatically,control of the selectivity and gain in such a manner that when the gainis at a maximum the selectivity is at a maximum and when the gain is ata minimum the selectivity is at a minimum. As is well known selectivitycontrol of this nature is of considerable practical advantage for radioreceivers, for in general maximum selectivity is required when thereceiver is operated to pick up signals from a remote or weaktransmitting station-that is to say, when the receiver gain is at amaximum.

According to this invention, a carrier frequency thermionic valvecircuit arrangement of the reaction type comprises a plurality of tunedvalve amplifiers in cascade, and there is inserted in series with theanode-cathode space of one of said amplifiers, an impedance, preferablya tuned impedance from which feed back voltage is taken,

said feed back voltage being superimposed upon voltage to a precedingvalve. Preferably, means are provided for adjusting the steady bias ofthe 4 preceding valve to which the feed back voltage is applied, and theinserted tuned impedance is on the cathode side of the valve with whoseanodecathode space it is in series.

Although not limited to its application thereto the invention isparticularly suitable for use with, and gives maximum advantage whenapplied to, the intermediate frequency amplifiers of superheterodynereceivers.

. The invention is illustrated in the accompanying drawing which showsdiagrammatically one embodiment thereof.

Referring to the drawing the intermediate frequency amplifier of aconventional superhetero- 5 dyne receiver (the rest of the receiver isnot shown) comprises at least two fixedly tuned amplifier valves I, 2,in cascade. These valves are preferably radio frequency pentodes (andare shown as such) though other types of valve may be employed. In thedescription which follows 5 it will be assumed that radio frequencypentodes are in question.

Input signals from the frequency changer (not shown) of thesuperheterodyne receiver are applied, for example through a suitablecoupling 10 condenser E, to the innermost or control grid 4 of the firstradio frequency pentode l, and to this grid is connected one end of aparallel tuned input circuit L1 C1. The other end of this parallel tunedcircuit is connected to an adjustable tap- 15 ping point 5 upon apotentiometer resistance 6, one end of which is connected to thenegative terminal I of the common source (not shown) of the anode supplyfor the receiver, the other end of the resistance 6 being connected tothe nega- 20 tive terminal 8 of a source (not shown) of bias potentialwhose positive terminal is connected to the terminal 7 which isgrounded. The screen grid 9 of the first pentode l is positively biased,as in the usual way, through a lead it from a source, 25 not shown, andthe outermost or suppressor grid II is connected to the cathode l2 alsoas in the usual way. The anode I3 of the first pentode is connected tothe positive terminal i l of the source of anode potential through aparallel 3 0 tuned circuit L2 C2 which is identical with the tunedcircuit L1 C1, and the said anode i3 is also connected through acoupling condenser i 5 to the innermost or control grid [6 of thesecond. pentode 2. This grid I6 is connected to the cathode of valve 2through a resistance [8 in series with a negative bias potential sourcel9, and this connection provides a partial anchoring of the said grid tothe cathode as regards high frequency potentials. Though theoreticallyit is better to anchor the grid Hi to the cathode ll it is often moreconvenient, in practice, to anchor the said grid to ground. Suchanchoring of the grid I6 to ground would introduce negative feed-back.from anode to grid in the valve 2, but this is of small importance sincesuch negative feed-back would be overwhelmed by the overall positivefeedback from tube 2 to tube I. The screen grid 20 of the pentode 2receives positive potential as in the usual way.

The suppressor grid 2| is connected to the cathode ll also in the usualway. The anode 22 of the pentode 2 is connected to the positive terminalof the source of anode potential through a parallel tuned circuit L3 C3which is identical with the two parallel tuned circuits alreadymentioned. The anode end of this parallel tuned circuit is coupled tosucceeding amplifiers, not shown-for example, to further intermediatefrequency stages or to a demodulating detector--for example, by acondenser 23. The cathode Il of the second pentode is connected to thenegative terminal I of the anode source through a fourth parallel tunedcircuit L4 C4 which is identical with the three parallel tuned circuitsalready referred to, the cathode end of this fourth parallel tunedcircuit being connected also to the cathode I2 of the pentode l. AIcy-pass condenser 25 is connected between the terminal 5 and themovable point 5 on the potentiometer resistance 6.

Since the two cathodes I2, H, are connected together and to one end of afeedback circuit constituted by the parallel tuned circuit L4 C4,feedback potentials will be applied to the oathode l2, and accordinglysuch feedback potentials will alter the grid-cathode potential of thepentode I in such manner as to assist input signals and provide additivereaction. If the control grid I6 of the pentode 2 is anchored relativeto its associated cathode, and not relative to ground, the feedbackpotentials will not affect the second valve, though, as above stated,even if the grid it is anchored to ground, such negative feedback as isthereby produced will be swamped. All four parallel tuned circuits areidentical, and for an intermediate frequency amplifier designed tooperate at a million cycles per second a value of 2 microhenries for theinductance and .012 microfarads for the capacity in each of the fourparallel tuned circuits have been found to give satisfactory results.

L4 C4 functions as the impedance disposed in the space current paths oftubes l and 2. Across this impedance Li C; is developed the feedbackvoltage. The small alternating current component of the space current oftube I develops across L4 C4 at resonance, a voltage which is impressedon the input electrodes of tube I in degenerative phase. The alternatingcurrent component of the space current of tube 2 is larger due toamplification, and by virtue of the phase reversal due to the resistancecoupling, there is developed across L4 C4 at resonance a larger voltagewhich is impressed on the input electrodes of tube I in regenerativephase. It can be shown that increasing the product of the gains of tubesI and 2, or the impedance of L4 C4, will increase the regenerativefeedback. Hence, by increasing the gain of tube I the regenerativefeedback across L4 C4 increases, and accordingly increased selectivityresults. Such gain increase, by shifting tap 5 to decrease the negativebias of grid 4, is employed when receiving weak signals; during weaksignal reception increased selectivity being desired.

For intermediate frequency amplifiers designed to operate at two millioncycles per second, the following values have been found satisfactory ineach casez-inductance value 2.53 microhenries and capacity value .0025microfarads.

With an arrangement as above described, it is possible to adjust thecircuit constants in such manner that when the movable tapping point 5is at, or near, the relatively positive end I of the potentiometerresistance 6, the self-oscillation point is just about reached, and inan arrangement so designed very high selectivity and gain will beobtained if the potentiometer tapping point be moved slightly in thenegative direction from the self-oscillation point. By moving thetapping point still more negatively both gain and selectivity may bereduced.

In place of using a potentiometer to give manual control in this way, anequivalent result, namely alteration of the D. C. grid bias upon thecontrol grid of the first pentode, may be obtained automatically independence upon received signal strength by any known automatic volumecontrol circuit connected and arranged to provide a D. C. bias potentialdepending upon received signal strength. Control can also be exercisedby varying the bias potential on the second grid, or alternatively byleaving the grid bias potentials fixed and varying the feedbackimpedance by means of a variable shunt resistance 3!! across the circuitL4 C4.

The following part theoretical analysis will assist in an appreciationas to the reasons for the satisfactory operation of a circuit as abovedescribedz-It is known that if an amplifier system be so arranged that afraction of the output voltage E be fed back to the input side, then, ife be the input voltage and the amplification factor, and if the fractionof the output voltage fed back be B,

that is to say, the effective amplification There are known reactiontype circuits where in, in order to secure stable amplification andrelative freedom from distortion, the co-efiicient B is made negative;that is to say, the feedback voltage is made of such sense as to reducemagnification. It is to be noted that in a circuit according to thepresent invention, and as above described, B is made positive, but theadjustments and design should be such that the product 3 is less thanunity, for when this product becomes equal to unity the oscillationpoint is reached. So long, however, as [LB is below unity the circuit isstable. If B is much less than unity, the effective amplification isonly slightly increased by reaction, and it will be noted that in thecircuit described B can be increased up to the limit set by theoscillation point; this increase being effected by moving the adjustabletapping point on the potentiometer towards the relatively positive endof the potentiometer resistance, the factor a being thus altered and thefactor B remaining constant.

The four parallel tuned circuits in the hereinbefore specificallydescribed embodiment of the invention are all sharply tuned and sincethey are identical the factor B will be equal to onehalf. With the thirdand fourth mentioned parallel tuned circuits L3 C3 and L4 C4 identicalthat is to say with B=.5 the oscillation point is reached when l=2, forwith these values of a and B the product ,uB unity. Preferably incarrying out this invention the circuit constants (including the biasupon the second pentode) are so adjusted that :2 or thereabouts, and Bis selected at .5 or thereabouts.

A. practical advantage of this particular selection design lies in thefact that with these values of mu and B the ratio of inductance tocapacity in the tuned circuits is very small as compared with the rationormally used in radio receiving circuits. For example, whereas in anordinary receiver circuit for 1,000 kilocycles 160-200 microhenries isconsidered a normal value of inductance for a tuned circuit, theinductance in each of the tuned circuits in the specifically describedembodiment of this invention may be, as stated, only 2 microhenries andthe capacity .014 microfarads. This low inductance-capacity ratiocontributes largely to the smooth and satisfactory working of thearrangement and because the tuning capacities are relatively large,effects due to stray capacity and residual Miller effect capacities aremuch reduced. It is also possible to design the circuits to have a highso-called Q value (resistance divided by inductive reactance), and asthis Q value determines the rate at which the amplification decreasesfor alteration of the frequency on either side of that frequency atwhich maximum amplification occurs (resonance), good selectivity isobtained. Even if there were no feedback, the initial conditions wouldbe such as to result in good selectivity.

Although in the specific embodiment of the invention above described andillustrated the circuits L1 C1, L2 C2, L3 C3, and L4 C4 are allidentical this is not an essential feature. For example, a convenientand stable embodiment is one wherein the circuit L4 C4 is shunted by asuitable resistance and in some cases the said circuit L4 C4 may bereplaced by a resistance. It is owing to the fact that best results areobtained with the use of large condensers in the tuned circuits that theinvention is most advantageously applicable to intermediate frequencyamplifiers operating at frequencies of the order of 1,000 to 2,000kilocycles.

In this connection it may be noted that it is very difficult with knownarrangements to design an intermediate amplifier of the ordinary typeand working at about 1,000-2,000 kilocycles which will have sufiicientselectivity to give two channel separation in a broadcast receiver withthe present spacing of broadcast transmitters in accordance withinternational agreement. With a circuit as herein described, however, ithas been found possible to obtain two channel separation quite easilyand in fact in many cases single channel separation can be obtained.These facts together with the large gain which is obtained at maximumselectivity make the invention very advantageous for use insuperheterodyne receivers. The invention is, however, not exclusivelylimited to superheterodyne receivers nor indeed to fixedly tunedamplifiers, for the described circuit arrangement may be modified toconstitute a tunable amplifier by making the tuned circuits adjustableas to their natural frequency and gang-controlling them.

What is claimed is:

A carrier frequency thermionic valve circuit arrangement of the reactiontype comprising a first valve having a carrier frequency tuned circuitin its control or input grid circuit and a carrier frequency tunedcircuit in its anode circuit, a second valve having its control or inputgrid coupled to the anode of said first valve and a carrier frequencytuned circuit connected between its anode and a source of anodepotential therefor, said second valve having an inserted impedanceconnected in series between its cathode and the negative terminal of thesource of anode potential therefor, said negative terminal alsoconstituting a point upon the grid circuit of the first valve wherebyvoltage set up across said inserted impedance is fed back to the controlor input grid of the first valve, means for adjusting the control orinput grid bias of said first valve, said inserted impedance being atuned circuit substantially identical with the other three tunedcircuits.

NOEL MEYER RUST.

